There is a rapidly growing demand for sources of exceptionally high purity quartz (SiO.sub.2), particularly in respect to very low levels of alkali-metal impurities, such as sodium, potassium, and lithium ions. Such high purity quartz is needed to prepare quartz frequency and timing control devices for electronic applications, as high purity raw materials for optical fibers, to prepare fused quartz optical grade glass such as for halogen vapor lamps, and to prepare crucibles and other quartz apparatus for making high purity silicon crystals for transistors, integrated circuits, and other electronic and semi-conducting devices such as photovoltaic cells.
There is an extremely limited supply, far less than is required for the above needs, of naturally occuring quartz crystals and quartz sands of sufficient purity. To meet the rapidly expanding needs in adequate quantity it will be necessary to develop techniques for purifying existing quartz raw materials to meet these needs.
Although quartz is commonly found in nature, either in a relatively pure state as in naturally occuring sand or in the form of complex ores such as pegmetites, granites, flints, shales, and many others, it is almost always associated with substantial impurities consisting chemically of A1.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O, Li.sub.2 O, Fe.sub.2 O.sub.3, FeO, TiO.sub.2, and ZrO.sub.2.
These impurities are usually present as distinct (i.e. non-quartz) minerals such as feldspar (complex K, Na, Li aluminosilicates), micas, garnets (iron-containing aluminosilicates), Zircon, ilmenite, and many others.
These impurities occur in the following modes:
(1) As loosely associated minerals, not chemically bonded to the quartz crystal;
(2) As mineral fragments which are chemically and physically bonded to the quartz crystal at their surfaces;
(3) As minerals which are occluded within the quartz grains or surrounded by an aggregate of quartz crystals bonded to one another; and
(4) As interstitial ions substituted within the quartz lattice itself. Chiefly these are as Al.sup.+3 and Fe.sup.+3 substituted for Si.sup.+4 in the SiO.sub.2 three dimensional lattice. When this occurs they are usually accompanied by a Li.sup.+4, K.sup.+1, Na.sup.+1, or H.sup.+1 ion to maintain the electrical neutrality of the SiO.sub.2 lattice.
In an attempt to remove these impurities from the quartz, a variety of techniques have been commonly employed. Where different types of impurities were present, a number of these techniques were used in sequence.
For example, a typical quartz purification procedure will usually include crushing, grinding, washing and size classification by air classification, sedimentation or screening. These steps separate physically associated impurities and water-soluble impurities and give a relatively uniform and small particle size material for further operations.
Next, heavy liquid separations, or a sequence of selective froth flotations employing different flotation agents, pH's and ion-adsorption salts such as fluoride will be employed to further separate physically associated minerals such as feldpars, micas, and garnets from the quartz crystals.
This may be followed by various magnetic separation methods for further removal of garnets and other magnetic impurities.
The above techniques will remove the majority of physically associated impurities but are not effective in removing chemically bonded impurities, occluded impurities, or particularly in removing interstitial impurities which are part of the quartz lattice itself.
Consequently, most procedures will next employ chemical reaction techniques such as leaching with aqueous caustic solutions, aqueous acidic solutions of various sorts, or high temperature (600 to 1100.degree. C.) reactions with various gaseous reagents. Included among the latter have been chlorinating acidic gases such as SO.sub.2, SO.sub.3, HCl, etc., and reductive chlorinative agents such as phosgene, carbon plus chlorine, HCl and the like. See, for example, G.D.R. 160,967, wherein the chlorinating agents mentioned include HCl, Cl.sub.2, Cl.sub.2 /CO mixtures, CCL.sub.4, and polyvinyl chloride, all in the presence of air; Japanese Patent Application Showa 61-106317, which discloses chlorination via insitu oxidation of HCl/air mixtures; and G.D.R 120,860, which discloses chlorination with aqueous HCl and aqueous HCl/HF mixtures.
The purpose of these procedures is to attempt a selective reaction, i.e., the user hopes that the rate of leaching or gaseous attack by reagents such as aqueous NaOH, HF, or gaseous phosgene or HCl with impurities such as feldspar, mica, garnet, etc., will be sufficiently more rapid than the reaction with quartz to accomplish selective purification.
These selective chemical attack procedures have been only moderately successful in removing chemically-bonded impurities adhering to the surfaces of the quartz crystals but have not in general been successful in removing interstitial (i.e. lattice-held) impurities. This was to be expected since it is clear that any reagent functioning by a chemical reaction mechanism would have to also react with the SiO.sub.2 matrix itself in order to reach interstitial impurities. It is clear that if this occurred it could only do so by destroying the very SiO.sub.2 matrix which is the desired end product.
The achievement of high levels of purity from alkali metals via recrystallization of quartz from purified raw materials in computer controlled recrystallization vessels is described in an article by Aulich et al., J. of Material Science, (1984) p 1710. These authors used purified quartz to prepare and spin high purity glasses which were subsequently acid leached with hot aqueous HCl to leave a matrix of silica of high purity. This method is likely to be extremely expensive.
Also, UK patent application G.B. 2166434H discloses preparation of high purity vitreous silica by application of a polarizing D.C - potential across the crucible wall of from 10 to 1000 volts per millimeter of wall thickness at temperatures of from 800.degree.-2000.degree. C. Substantial purification of alkali metal impurities is claimed. Again, this method is likely to be extremely expensive.
Finally, GDR 160,967 discloses a process for purifying quartz raw materials by subjecting quarts raw materials contaminated with mica and/or chlorite-group minerals to a mechanical preparation and treatment with an aqueous HCl or aqueous HCl/HF mixture, followed by a heat treatment at 800.degree.-1200.degree. C., optionally in the presence of HCl gas, followed finally by an HF leach. Ultimately, the levels of alkali metal impurites are reduced but the levels are still too high. Thus, the quartz produced by this process still contains too much alkali metal impurity to be used in applications demanding exceptionally high purity quartz.
Accordingly, it is an object of the present invention to provide a process for obtaining highly purified quartz, which is substantially free of interstitial impurities, particularly interstitial alkali metal impurities.
It is also an object of the present invention to provide a process whereby such highly purified quartz can be obtained without causing damage to the SiO.sub.2 matrix.
It is further an object of the invention to provide a process whereby such highly purified quartz can be obtained efficiently and at a reasonable cost.